1. Field of the Invention:
The present invention relates to a process and a device for cooling the internal wall of a pipe made of plastics material during its extrusion.
2. Description of the Prior Art:
Modern manufacture of pipes made of plastics material by extrusion demands high-output extruders. Nevertheless, in order to produce large quantities of high quality pipes it is essential that the pipes are cooled during their extrusion in the most intensive and effective manner possible.
Conventional cooling using a water jet on the external surface is no longer suitable, given that the cooling is limited by the length of the cooling distance. The heat to be dissipated also depends on the thickness of the pipe wall and on the extrusion speed.
A solution has therefore been sought based on cooling via the inside of the pipe. Proposals based on the use of counter-current air or water or based on indirect cooling give results which are less than satisfactory.
Internal cooling by the dispersion of water has proved to be more promising. For this purpose small water droplets are sprayed in the form of a mist onto the internal wall of the pipe. From the heat transfer point of view the spraying of droplets represents a good solution, given that the quantity of heat necessary for the evaporation of water (latent heat) and therefore the heat removed from the pipe to be cooled, is about five times the quantity of heat which is removed by the water in a convective process.
Various modifications of this type of cooling are described in German Patent Applications DE-A 2455779, DE-A 3241005 and DE-A 3414029. According to these modifications, the cooling agent is sprayed in the form of droplets via a fixed device comprising a central orifice or a plurality of orifices distributed around the circumference. The device is disposed fixed, and is fixed in relation to the tube which travels axially.
It has been found that this cooling system comprises disadvantages and shortcomings.
In practice an important phenomenon which affects the cooling of pipes stems from the thermal conduction of the constituent material. The coefficient of thermal conduction of plastic materials is low. It follows from this that heat located at the surface or near the surface will be easily removed, but heat located at some depth in the pipes requires a period of time in order to reach the surface. Spraying droplets as proposed in the prior art only enables heat to be removed from the surface of the pipe.
Moreover, if a film of water is atomised onto the hot wall of the extruded pipe, part of the water is evaporated but the major part of the dispersed water runs from the pipe wall towards the bottom and converges in this region. The water then only contributes to the internal cooling of the pipe in this region. The cooling of the pipe is asymmetric. Furthermore, it has been noted that water droplets dispersed on a pipe made of plastic material preserve their form, given that the wetting between the water and the plastic material is poor, particularly when high density polyethylene is used. This phenomenon also impairs the efficacy of cooling with the aid of dispersed droplets.